This invention relates to an antenna unit and, in particular, to an antenna unit for an ultra wideband (UWB).
The UWB technology means an ultra wideband radio technology and is a broad term representative of a radio transmission system that occupies a bandwidth of at least 25% of a center frequency or a bandwidth wider than 1.5 GHz. Briefly speaking, the UWB technology is a revolutionary radio technology for carrying out communication using short pulses (typically having a pulse width of 1 ns or less) of an ultra wideband.
A key difference between the UWB technology and the traditional radio technology is presence or absence of a carrier wave. In the traditional radio technology, a sinusoidal wave having a certain frequency, called a carrier wave, is modulated in various manners to transmit and receive data. On the other hand, the UWB technology does not use the carrier wave but uses the short pulses of an ultra wideband as described above.
The traditional radio technology uses a narrow frequency band. This is because communication using a narrow frequency band allows an effective use of a radio wave which is a finite resource.
As understood by its name, the UWB technology uses an ultra wide frequency band. Nevertheless, the UWB technology occupying an ultra wideband attracts increasing attention. The reason resides in output energy at each frequency.
In the UWB technology, the frequency band is very wide but output power at each frequency is very low. Since the output power has a very low level below a noise level, an interference between UWB communication and other radio communications is extremely small.
On the other hand, the UWB communication extends over an ultra wideband which overlaps frequency bands of existing radio communication services. Therefore, the band for the UWB communication is limited to a range from 3.1 GHz to 10.6 GHz at present.
In the meanwhile, an antenna basically utilizes a resonance phenomenon. The antenna has a resonance frequency determined by its length. In the UWB communication including a number of frequency components, it is difficult to cause a resonance. As a frequency band of a radio wave to be transmitted is wider, it is increasingly difficult to design the antenna.
For example, a patch antenna is known as a compact antenna. As an example of the patch antenna, a compact flat patch antenna is disclosed, for example, in Japanese Unexamined Patent Application Publication (JP-A) No. H7-94934. The flat patch antenna is excellent in portability, high in frequency temperature characteristic, suppressed in variation of the resonance frequency, and excellent in reliability. A patch antenna unit adaptable to a plurality of frequencies is disclosed, for example, in Japanese Unexamined Patent Application Publication (JP-A) No. H10-190347.
However, the patch antenna does not cover a wideband and is therefore unsuitable for use as a UWB antenna.
In the meanwhile, TAIYO YUDEN CO., LTD., Tokyo, Japan has successfully developed a very small ceramic chip antenna for UWB which is recognized as a next-generation technology that will simultaneously achieve high-rate data transmission and low power consumption in the field of short-range radio communication. The ceramic chip antenna has a size of 10 mm×8 mm and a thickness as small as 1 mm. By the development of the ceramic chip antenna, the UWB technology traditionally limited to military applications is now released for commercial use, for example, high-speed connection of data between digital equipments such as a PDP (Plasma Display Panel) television and a digital camera. It is therefore possible to miniaturize various equipments, including mobile equipments.
Such a UWB antenna may be used for various applications such as Bluetooth (registered trademark) and wireless LAN (Local Area Network).
Bluetooth is an open standard for an advanced technology for realizing wireless data and voice communication in a relatively short range between desktop and notebook computers, PDAs (Personal Digital Assistants), mobile telephones, printers, scanners, digital cameras, and home electric appliances, and so on. Bluetooth is operable using a radio wave in a worldwide available 2.4 GHz band and, therefore, can be used throughout the world. Briefly speaking, use of Bluetooth makes it possible to connect digital peripheral devices without cables. Therefore, any trouble associated with cable connection is a matter of the past.
The wireless LAN is a LAN using a transmission path, such as a radio wave or an infrared ray, except a cable and a wire.
As described above, an existing antenna such as a patch antenna is disadvantageous in that it is difficult to cover a wideband and waveform distortion (waveform broadening or widening) occurs. Further, an antenna characteristic is excellent if a voltage standing wave ratio (VSWR) is as low as possible. However, the existing antenna can not lower the VSWR in a high-frequency range. In other words, the VSWR inevitably becomes high in a high-frequency range.